1. Field of the Invention
The invention relates to membranes which have electron and oxygen ion conducting properties, the method of preparation thereof and the applications thereof, in particular for the oxidative dehydrogenation of alkanes into alkenes.
2. Description of the Related Art
Oxidative dehydrogenation methods which allow saturated organic compounds to be converted into unsaturated compounds are well known. A large number of attempts to improve methods of this type have been made over recent years in order to increase in particular the conversion rates and the selectivity with regard to the desired products.
Ethylene is produced industrially by means of thermal cracking of ethane, generally in the presence of water vapour, which consists in a pyrogenic reaction of ethane at high temperatures in the order of 850° C. At the present time, the selectivity rate in terms of ethylene obtained according to this method is not higher than 80% and the yield in terms of ethylene is in the order of 60%. Although an increase in the temperature allows higher conversion rates to be achieved, this increase, however, brings about a reduction of the selectivity by promoting the formation of secondary products and their decomposition in the form of coke. The depositing of coke on the walls of the pipes of the installation principally brings about two harmful effects; on the one hand, it results in pressure drops through the pipes of the installation and, on the other hand, it offers resistance to the transfer of heat to the hydrocarbon fluid.
The formation of coke constitutes one of the main factors limiting cracking operations.
In order to attempt to overcome this limitation, methods for oxidative dehydrogenation of ethane have been developed.
There are substantially two different types of method for oxidative dehydrogenation: catalytic methods which use catalyst beds and those which use mixed conducting membranes.
The main reactions which are involved in the catalytic method are as follows:C2H6C2H4+H2 H2+½O2→H2OC2H6+ 7/2O2→2CO2+3H2O
Catalysts which allow yields for converting ethane into ethylene in the order of 45-50% to be achieved at temperatures in particular less than 700° C. have recently been described.
In this manner, Thorsteinson et al. (J. Catal., 52, 1978, 116) obtained an ethylene yield of 25% with a selectivity of 90% using the catalyst MoVNbTeOx at 350° C.
Higher yields of between 44 and 50% have also been reported by Ji and Liu et al. (L. Ji, J. Liu, Chem. Commun., 1996, 1203) and Velle et al. (O. J. Velle, A. Andersen, K.-J. Jens, Catal. Today, 6 (1990) 567) with the catalysts La/CaO, Li/La/CaO and SrCe0.5Yb0.5O2.75 in a fixed bed reactor, but at temperatures in the order of from 600 to 700° C.
However, in order to achieve a yield of this type, it is necessary to operate in an oxygen-enriched atmosphere, which brings about an increase in operating costs.
Generally, although they allow the problem of coke formation to be overcome, catalytic methods for oxidative dehydrogenation are nonetheless difficult to use on an industrial scale at the present time. They have a number of disadvantages, in particular yields which are lower than those obtained using commercial methods, the consumption of the hydrogen produced, the formation of oxygenated secondary products, the inflammable nature of the reaction mixture, the requirement for oxygen enrichment and the costs which result.
The methods for selective oxidative dehydrogenation of ethane into ethylene which use dense oxygen ion conducting membranes have been found to be much more promising.
Dense conducting membranes are membranes which are capable of selectively transporting oxygen ions at temperatures which are generally higher than 600° C., most often between 700° C. and 1200° C.
The different types of dense membrane for conducting oxygen include in particular membranes composed of solid electrolyte, the ion type, and mixed ion-electron conducting membranes based on multimetal oxides.
Membranes which are formed from solid electrolyte comprise inorganic oxides, typically oxides of calcium or zirconium, which are stabilised by yttrium (YSZ) and generally have a perovskite or fluorite structure.
These membranes of the ion type transport only oxygen ions and require the application of an external electric field in order to maintain an electron flow through the membrane and thus the process of ionisation-deionisation.
Mixed ion-electron conducting membranes, in particular monophase membranes which are constituted by multimetal oxides, are, however, capable of transporting both oxygen ions and electrons without it being necessary to apply an external electric field.
The driving force which allows oxygen to be transported in mixed conducting membranes is based on a partial pressure difference of O2 applied at one side and the other of the membrane. Owing to its non-porous structure, the membrane prevents any gas molecule from passing through directly. Only the oxygen ions are able to selectively migrate. The dissociation and ionisation of the oxygen is brought about in contact with the surface where the partial pressure is highest (cathode), by means of electron capture. The charge flow of the oxygen ions O2− is compensated for by a simultaneous flow of electrons in the opposite direction. When the oxygen ions reach the side of low partial oxygen pressure, that is to say, the side of the permeate (anode), the oxygen ions donate their electrons and recombine in order to regenerate molecular oxygen which is released into the permeation current.
These membranes, which are used as membrane reactors for the oxidative dehydrogenation of ethane, selectively transport oxygen as far as the anode where they react rapidly with ethane in order to form ethylene.
The transport of oxygen through these conduction membranes is a controlled process which is substantially dependent on two factors: the kinetic surface limitations and the limitations of volume diffusion.
The kinetic surface limitations are linked to the plurality of steps which are involved during the conversion of the oxygen molecules in a gaseous phase in the charge current into mobile oxygen ions and, conversely, of the mobile oxygen ions into oxygen molecules at the side of the permeate of the conducting membrane or during the reaction of the oxygen ions with the reactant gas.
The limitations of volume diffusion are linked to the diffusion of the oxygen ions and electrons at the inner side of the membrane.
These two limiting factors are dependent in particular on the partial pressure gradient in terms of O2 at one side and the other of the membrane, the operating temperature and the thickness of the membrane.
Work has been carried out which is intended to optimise the efficiency levels of these membranes, which are advantageous in particular when used as membrane reactors for the oxidative dehydrogenation of ethane into ethylene.
A first approach involves seeking multimetal mixed conducting oxide compositions which have good intrinsic properties for transporting oxygen.
Document Haihui Wang et al., Chem. Comm., 2002, 1468-1469, describes a reaction for oxidative dehydrogenation of ethane into ethylene in a membrane reactor which is constituted by Ba0.5Sr0.5Co0.8Fe0.2O3-δ.
The ethylene is obtained at a yield of 15%, a conversion rate of 18%, a selectivity of 90% at 650° C. and an oxygen permeation flow of 0.36 mL·min−1 cm−2.
In the document H. Wang et al. (Catalysis Letters, 2002, vol. 84, Nos 1-2 pp. 101-106), the oxidative dehydrogenation reaction of ethane in the Ba0.5Sr0.5Co0.8Fe0.2O3-δ membrane reactor is examined at 800° C. A maximum yield of 67% is obtained with a selectivity with regard to ethylene, a conversion rate and a permeation flow of oxygen of 80%, 82% and 1.6 mL cm−2 min−1, respectively.
Another approach involves reducing the kinetic surface limitations by associating a catalyst which promotes, for example, the dissociation of gaseous oxygen into mobile O2− ions.
Document EP 0 663 231 (Air Products and Chemicals, Inc.) thus describes a membrane which comprises a porous mixed conducting layer of multimetal oxide, one surface of which layer is covered with a catalyst and the other is in contact with a dense mixed conducting multimetal layer. The catalyst comprises a metal or a metal oxide which catalyses the dissociation of oxygen molecules into ions and/or the association of oxygen ions into molecular oxygen.
The two membranes which are set out by way of example in this patent application use a platinum-based catalyst at a ratio of 10 mg/cm2 of surface-area. The use of an expensive precious metal in a substantial quantity as a catalyst renders the use of these membranes on an industrial scale improbable.
Document WO 99/21649 describes a catalytic membrane reactor which comprises an oxidation zone and a reduction zone which are separated by a membrane which is impermeable to gases, and which has an oxidation surface in contact with the oxidation zone and a reduction surface which is in contact with the reduction zone, a layer of adhesive catalyst on the oxidation surface of the membrane and, optionally, a three-dimensional catalyst in the oxidation zone. However, the catalysts are in the form of a continuous coating or layer.
Furthermore, only the catalysts La0.8 Sr0.2 MnO3 and cermet of Ni on La0.8Sr0.2 MnO3 are actually envisaged. There is no mention anywhere of dispersed particles based on magnesium or noble metals.
These membranes are difficult to use and attempts have been made to produce membranes which are effective, less costly and in particular, much easier to use.